This invention relates to new classes of polysilahydrocarbons and methods of making them. The invention also pertains to the application of such polysilahydrocarbons as synthetic lubricant base stocks. The term "polysilahydrocarbon" is used here to represent compounds containing more than one silicon atom in a molecule, including di-, tri-, tetra-, penta- and hexasilahydrocarbons depending on the specific number of silicon atoms in a molecule.
A wide variety of synthetic lubricant base stocks have been available for some time. These include esters, silicones, polyalphaolefins, trialkylated cyclopentane (Pennzane.RTM.) and the like. These base stocks are prepared by specific chemical synthesis methods rather than by refining of crude petroleum-based oils, and they offer distinct advantages over conventional mineral oils. These synthetic lubricant base stocks are particularly useful in aerospace and military applications which require performance at extremes of temperature, vacuum and hostile environments.
Among the various synthetic lubricant base stocks, tetraalkylsilanes, commonly referred to as silahydrocarbons, offer particular promise. A number of silahydrocarbons, particularly the monosilahydrocarbons, are described in U.S. Pat. Nos. 4,711,966, 4,595,777, 4,578,497 and 4,572,791 and references contained therein. The monosilahydrocarbons described by C. E. Snyder, Jr., et al., ASLE Transactions, Vol. 25, No. 3, pp. 298-308 (1982) have been shown to be superior to other classes of synthetic lubricant base stocks. For instance, they are superior to silicates in hydrolytic stability, lubricity and bulk modulus. They are also superior to polyalphaolefins in viscosity-temperature properties and thermal stability. The desirable effects of the silicon atom in a saturated hydrocarbon structure, as it exists in the monosilahydrocarbons referred to above, is well known to those familiar with the art. However, in applications requiring extremely low volatility, such as applications in the near-vacuum of space, these monosilahydrocarbons are not suitable. Useful monosilahydrocarbons are of relatively low molecular weight and would tend to "boil off" in the near vacuum of outer space. Therefore, higher molecular weight materials are required. One way of increasing the molecular weight of the monosilahydrocarbon is to increase the number of carbon atoms in the four alkyl residues attached to the silicon atom. This would lead to a very low ratio of silicon to carbon. It is likely that such materials would resemble high molecular weight hydrocarbons and would be waxy in nature rather than liquid. An alternate method of increasing molecular weights and thereby reducing volatility would be to have more than one silicon atom distributed in a high-molecular weight hydrocarbon structure. Such a structure would have the desired high molecular weight and at the same time possess the desirable liquid properties associated with the silicon atoms when present in a hydrocarbon structure.
Work in this direction has resulted in a series of compounds classified as di-, tri- tetra- and pentasilahydrocarbons. For example, some disilahydrocarbons are described in U.S. Pat. Nos. 3,296,296 and 3,347,897. For the preparation of some trisilahydrocarbons, see U.S. Pat. Nos. 4,788,312 and 3,580,940. U.S. Pat. No. 5,026,893 describes the synthesis and properties of a series of tetra- and pentasilahydrocarbons of the following general structures I and II, respectively: EQU R.sup.1 --Si[--R--Si(R.sup.2 R.sup.3 R.sup.4)].sub.3 (I) EQU Si[--R--SiR.sup.2 R.sup.3 R.sup.4 ].sub.4 (II)
where R.sup.1, R.sup.2, R.sup.3 and R.sup.4 represent alkyl groups having from one to 20 carbon atoms, and --R-- represents an alkylene group having from three to 10 carbon atoms. While these inventions provide useful materials suitable for application in the near-vacuum environments of space, improvements in their properties and methods of preparation are desirable. For instance, base fluids having improved compatibility with known additive materials would be advantageous. In addition, the methods of synthesizing compounds having the general structures I and II involve multiple-step processes, which have the usual limitations and inefficiencies associated with such processes. More particularly, a Grignard reaction involving nine to 12 chlorines can result in an incomplete reaction leading to a product containing residual chlorine. Furthermore, a hydrosilylation step using a terminal olefin which can undergo isomerization leading to inactive internal olefins can result in an incomplete reaction. When high molecular weight terminal olefins isomerize, the internal olefins formed do not undergo hydrosilylation, resulting in high molecular weight impurities which are difficult to remove. Such impurities, when present in the lubricant, can outgas in space and result in contamination of sensitive components on board the spacecraft. Preparation of a material of high purity by this method is often difficult. Purification of the final product is of particular significance since these materials have extremely low vapor pressure and as a result, are not amenable to conventional means of purification such as distillation.
Therefore, there is a need for polysilahydrocarbons which have low volatility and good viscosity/temperature and wear properties. There is also a need for simple methods of synthesizing polysilahydrocarbons which are capable of providing high-purity products in high yield.